We have studied the regulation of intracellular enzyme level and protein turnover in Escherichia coli. We have proposed that the degradation of glutamine synthetase (GS) in E. coli is a two-step process involving inactivation followed by proteolysis (Levine, R.L. et at., (1981) Proc. Natl. Acad. Sci. 78, 2120-2124). We have characterized the properties of the inactivation reaction and several enzymic mixed function oxidation systems which catalyze GS inactivation (Oliver, C. and Stadtman, E. R., Annual Report 1980-1981) and we have demonstrated that enzymes other than GS are inactivated in a similar manner (Fucci, L., Oliver, C., and Stadtman, E. R., Annual Report 1981-1982). As an extension of these observations, studies have been initiated to examine the possible physiological role of bacterial enzyme inactivation by activated neutrophils which are capable of ingesting and killing bacteria. Other studies have been undertaken to isolate and purify an immunomodulator from activated macrophages. Techniques used in these studies have included pore gradient electrophoresis, isotopic labeling chromatographic techniques, enzyme assay, high performance liquid chromatography and tissue culture.